Recently, portable, cordless consumer electronic appliances are being rapidly developed. At present, there is an increasing demand for compact, lightweight batteries with high energy density as power sources for these electronic appliances. In particular, a growing market share is expected for lithium ion secondary batteries as power sources for notebook PCs, cellular phones, AV equipment and the like, since they have high energy density and high voltage. Nickel-cadmium storage batteries or nickel-metal hydride storage batteries using an alkaline aqueous solution as the electrolyte, which have been the mainstream, are being replaced with lithium ion secondary batteries.
As positive electrode active materials of lithium ion secondary batteries, lithium-containing composite oxides such as LiCoO2, LiNiO2, LiMnO2 and LiMn2O4 are used. These positive electrode active materials undergo repeated expansion and contraction when subjected to charge and discharge. At this time, destruction of the crystal structure or cracking of the particles is caused, resulting in a decrease in capacity or an increase in internal resistance during charge/discharge cycles. In response to this problem, it has been reported that the crystal structure is stabilized by partly replacing cobalt or nickel with other elements.
For example, it has been reported in Japanese Patent No. 3162437, Japanese Unexamined Patent Publication Nos. Hei 5-242891 and Hei 6-168722 that the cycle characteristics and the safety are improved by partly replacing cobalt in the positive electrode active material with an element such as magnesium. However, while the deterioration of the cycle characteristics can be suppressed, it has been confirmed that when, for example, a charged battery is stored at 85° C. for three days, the amount of gas generated in the battery is relatively larger. Particularly, in square thin batteries and batteries having an outer jacket made of a laminated sheet, the strength of the case or outer jacket is low, so that the gas generation may lead to an increase in thickness and a decrease in capacity of the batteries. Although the cause of the increase in the amount of gas generation is uncertain at the moment, it is believed that since a positive electrode active material in which cobalt is partly replaced with magnesium has high electronic conductivity and the surface of the active material is active, its reactivity with a non-aqueous electrolyte increases, promoting decomposition of the non-aqueous electrolyte.